A communication channel such as a voice grade telephone line generally has a given capacity of a certain number of signals that it may carry. Demand for communication capabilities has increased substantially in recent years. Attempts to meet this increased demand have included increasing the number of communication channels (which can be very expensive), and increasing the capacity of each communication channel.
Various techniques have been implemented for increasing the capacity of a communication channel. Among these previously-used techniques are increasing the bandwidth of the communication channel, frequency domain multiplexing of the signals, and time domain multiplexing of the signals. Each of these previously-attempted techniques have encountered difficulties and have their drawbacks.
One type of communication channel for which an increase in capacity is frequently desired is a four-wire leased voice grade telephone line. Four-wire leased voice grade telephone lines typically connect two facilities at two different locations, each having a private branch exchange (PBX) as its switching equipment. The bandwidth of a typical leased line is 3.2 kilohertz (kHz), which is suitable for the transmission of a voice signal with a bandwidth of 200 Hz to 3400 Hz.
An object of the present invention is to increase the capacity of a communication channel, not by altering its bandwidth, but by applying two bandwidth-compressed signals to the single communication channel.
Another object of the invention is to modulate two or more signals onto carrier signals for transmission on a single communication channel so that the most important frequency components of the signals are near the center of the bandwidth of the communication channel.
An object of the invention is also to provide an improved single sideband signal generator that generates single sideband signals using less equipment than the prior art.